This invention relates to a novel process of producing a liquid developer for use in the development of an electrostatic latent image, and, more particularly to a convenient process for producing toner particles excellent in dispersion stability.
In general, liquid developers comprise a hydrocarbon medium of high insulation resistance and, dispersed therein, colored particles of several .mu.m or below in size provided with a positive or negative electric charge. In stabilizing a dispersion of such fine particles in non-aqueous media, utilization of the coulomb repulsion originated from the charge on the surface of each particle is not a dependable means to resort to, unlike the case of an aqueous emulsion; a generally effective way is to establish a protective adsorption layer of a polymer around each particle to take advantage of the steric repulsive effect between particles enveloped in such a layer.
As an example of a polymer suitable for forming the protective adsorption layer, there may be mentioned a graft- or block-copolymers having in the skeletal structure of its molecule both components soluble and insoluble in the dispersion medium. The ability to stabilize the dispersed particles, however, is comparatively sensitive to the composition, structure, and moledular weight of such a graft- or block-copolymer and it is very difficult in practice to synthesize a copolymer of controlled structure and molecular weight.
There is another process such as is disclosed in, for example, U.S. Pat. No. 4,081,391, which comprises introducing a polymerizable vinyl group into a precursor polymer, and polymerizing a monomer in the presence of the resulting vinyl-containing precursor polymer to produce a graft copolymer. In such a process, depending upon the concentration of the precursor polymer, there occurs crosslinking reaction among polymer molecules, resulting in indispersible particles.
Another necessary condition for the formation of a stable dispersion is associated with the particle size. In order that a liquid developers may remain as a stable emulsion without settling of toner particles during a long-term storage or in use, the particle size is required to be several .mu.m or less, preferably 0.3 to 0.4 .mu.m or less. As for the developing characteristics, a narrow particle size distribution is expected to be favorable for the uniform reproduction of an image. It is, therefore, an important problem for the liquid developer to control the particle size and its distribution.
In the conventional method of forming dispersed particles by the use of graft- or block-copolymers, the requirements for the particle size and its distribution can be met to a certain extent, in principle, by the control of the structure of graft- or block-copolymers, but the practicability is questionable.